HMGB1 and primary progressive multiple sclerosis: HMGB1 mRNA expression levels were determined by PCR in peripheral blood mononuclear cells (PBMC) of 29 healthy controls and 57 untreated MS patients (26 with relapsing-remitting MS - RRMS, 13 with secondary progressive MS - SPMS, and 18 with primary progressive MS - PPMS).